For the third month in a row the rain for the last month is reset at month change. It's set to the rain from the previous month. Like today, October 1, the total rain for September was reset to the total for August at midnight.

I enclose a screenshot that I took right before midnight September 30 (T6). It shows the monthly rain at 137.6mm. Then the same screenshot taken this morning, October 1 (E6). The rain for September has been reset to 47.6mm, the same as August. Notice that the rain for the year is correct and has not been changed at month change.

Mine seems to be all over the place as well, July & August have the same amount of rain 26.4mm, it looks like August should be 44.6mm, then September shows 44.6mm instead of 30.8mm. I will need to do some more checking. Using 10.37 S79 & windows 10 version 1803.